Photoreception occurs in the highly specialized rods and cones of our retina. These photoreceptors are morphologically divided into inner and outer segments. The inner segments contain much of the metabolic machinery necessary for providing energy and synthetic components to the cell. In contrast, the outer segments are specifically adapted for conducting phototransduction. Arrestin is known to function in phototransduction, quenching rhodopsin through binding to photoactivated rhodopsin after it has been phosphorylated, thus blocking transducin activation. With this role in mind, one would expect to find arrestin in the outer segments co-localized with rhodopsin. Instead, most of the arrestin is found in the inner segments of dark-adapted rod photoreceptors, and then migrates to the outer segments in response to light on a time scale of minutes when R*P quenching occurs on a time scale of milliseconds. At the same time that arrestin is entering the outer segments, transducin is leaving the outer segments and moving to the inner segments. How are these proteins moving? What function does this massive translocation serve in the photobiochemistry of the eye? The long-range goal of our research is to understand the mechanism and function of this light-driven protein translocation in photoreceptors. This proposal begins to answer these questions by defining the underlying mechanism regulating arrestin migration and identifying the molecular elements involved in this process. Based on preliminary data, our hypothesis is that arrestin and transducin translocate using active, motor-driven pathways and that these pathways are different for arrestin and transducin. The following Aims are addressed in this proposal: Aim #1 What is the mechanism by which arrestin moves between the inner segment and the outer segment? Aim #2 What is the domain(s) on arrestin that couples the protein for light-driven translocation? Aim #3 What element in the cytoskeleton does arrestin bind? Successful completion of these proposed experiments will provide molecular handles that can be used to determine the function of light-driven arrestin translocation. Further, this information will also identify molecular elements in which defects could have a potential pathological effect on vision.